Readonly ranges/lines in the editors

In the Cosmos DB extension exploration, documents are opened as JSON files, which can be edited  They may have user defined properties, as well as several database-defined properties. Editing these is possible, and _may_ be desired for the `_etag`, but in general will have no effect, be ignored, or break things in odd ways. On the other hand, it's not appropriate to omit these entirely, as it can be useful to reference or copy. It could be helpful to have readonly ranges within the editor to help users understand that changes made within these fields have no affect. Readonly ranges also came up a few days ago in the context of @JacksonKearl's search editor a few days ago.